THE VISUAL PIGMENTS 



10 min, the measurements {D-E, E-F, Fig. 7.1) showed that bleaching 

 of the photosensitive pigment and formation of a photoproduct had 

 occurred. This photoproduct was thermally unstable, as is shown by 

 the fact that the return measurements E-F did not retrace the path of 



T — r 



500 600 



Wave-length (m//) 



Fig. 7.2. Effect of storage on the difference spectra of frog visual cell 



suspensions. #, measurements made from short to long wavelengths; 



O, 'return' measurements. 



{Arden, 1954b) 



the outward ones, D-E. Three hours later the measurements H-I 

 and I-J were obtained. These showed that thermal decomposition 

 of the initial photoproduct had been completed by that time. 



EFFECT OF AGEING THE SUSPENSIONS 



In Fig. 7.2 the difference spectra for a fresh suspension are com- 

 pared with those obtained with suspensions stored for 24 hr at 2°C 



184 



